How do we read more selectively, and resist the temptation to waste our reading energy?
Trust your instincts.
I've always fancied that I had a pretty good instinct about what would be a good book or movie. I could pretty well judge a book or movie by its cover, and that's usually because the marketers who designed those book and movie covers were doing it pretty systematically, trying to target audiences that weren't me. With that in mind, I think we all have a pretty good record of judging things smartly based on a blurb or a few pages. So I'd encourage you to trust your instincts; even if someone is recommending something to you, take a look at it first and consider if it seems to be targeting you or not.
Make it harder to read junk.
That side of things doesn't address the temptations of the internet. In that case, we often know already that the post is going to be a waste of time, and yet we still click the link, read the article, and feel silly afterward. Why do we do it? We know that the video spoofing vintage breakfast cereal commercials isn't really going to better our lives or give us new insights into the world - so why, why do we watch it?
Sometimes we just want things easy; we want a laugh, or a little entertainment. There's nothing wrong with that; it just comes down to prioritizing and budgeting. In order to limit your intake of time-wasting articles, you can't count on your own self-control; it's a very finite resource, and it can be very fatiguing to rely on. So instead, you've got to throw up roadblocks for your reading. Don't automatically open your usual links at the start of the day. Avoid an RSS reader if you must until you've gotten some work done. Think of that stuff as a reward for work, not as the necessity before work. Don't even let yourself get on the computer right after school. Instead, sit at your table with a notebook and pen, enjoy your coffee or tea, and jot down what you're thinking. This might be a far gentler, more relaxing, and more productive way to start your day.